Presenter's biography

Biographies are supplied directly by presenters at OFFSHORE 2015 and are published here unedited

Dr. Wakui has been studying on wind power generation from the viewpoint of the system engineering approach. He is currently an associate professor at the department of mechanical engineering, Osaka Prefecture University. After getting the Ph.D. in the optimal design and control of stand-alone wind turbine generator systems in 2001, he spent 4 years at Waseda University and then transferred to the current university. The current his study focuses on the optimal control for floating offshore systems as well as grid-connected and stand-alone systems and the optimization of distributed energy supply systems.

Abstract

Floating offshore wind turbine-generator systems are expected to install in areas that have very deep waters. Although these systems require a cost reduction, sufficient strength design, and platform stability for commercial use, the establishment of the platform stability by a control approach contributes considerably to satisfaction of the other requirements. Previous studies on this field mainly focused on complicated control approaches, including a state feedback control, robust control, and active damper-based structure control, to tackle the trade-off between platform stability and power output fluctuation reduction. However, powerful computational resources and many additional sensors are indispensable for their implementation.

Approach

The present study develops a novel control parameter setting for a conventional feedback control of the rotor speed based on collective pitch angle manipulation to reduce the power output fluctuation as well as the platform motion. This development is conducted through a numerical analysis of a spar-type floating offshore system using the aeroelastic simulation model (FAST), observed high wind speed data, and irregular sea waves. The proportional-plus-integral action is employed for the rotor speed control. The proportional gain and integral time are parametrically varied on the basis of the frequency response characteristics, which are derived by system linearization.

Main body of abstract

First, the system performances at high wind speeds are analyzed in a wide range of the control parameters. As the proportional gain becomes larger and the integral time shorter, the power output fluctuation decreases while the platform pitching motion increases. This parametric analysis reveals that a novel control parameter setting, in which the proportional gain is large under a sufficiently long integral time, can reduce the power output fluctuation significantly and hardly increases the platform pitching motion. In this setting, the natural frequency of the rotor speed is higher than that of the platform pitching motion, but the damping coefficient is much larger than 1. The latter greatly counteracts the negative damping effect due to the pitch angle manipulation.
Second, the system performances in the derived setting and other conventional settings are compared. The conventional settings were developed by the NREL for onshore and offshore systems, where the natural frequency of the rotor speed is much higher than that of the tower motion and lower than that of the platform pitching motion to avoid the negative damping effect, respectively. In both the settings, the damping coefficient was set to about 0.7. The platform pitching motion and damage equivalent fatigue loads in the derived setting are almost the same levels as those in the offshore setting, but the rotor speed and power output fluctuations are significantly reduced. Moreover, the derived setting has a great advantage in all the system performances over the onshore setting.

Conclusion

The present numerical analysis found a novel control parameter setting for the rotor speed of the spar-type floating offshore system, which can reduce both the power output fluctuation and platform pitching motion. This good reduction is achieved by the high damping to the platform to prevent the negative damping effect, and the high natural frequency to improve the control performance of the rotor speed. This novel parameter setting for the conventional rotor speed control greatly contributes to the satisfaction of the above-mentioned requirements for floating offshore systems because there is no need to install powerful computational resources and additional sensors.

Learning objectives
The present study provides the new insights regarding the impact of the parameter setting for the conventional rotor speed control, especially high damping effect to the platform, on the performances of the spar-type floating offshore system. The dynamic characteristics of this floating system, including the generating performances and fatigue load characteristics as well as the power output fluctuation and platform motion, can also be learned through this parametric analysis.

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